Aims: This study was aimed to investigate mechanical properties (Hardness and Elastic Modulus) of three thermoplastic materials used in the fabrication of orthodontic aligners. Methods: Shore D hardness test was used to measure the hardness of three companies of hard thermoplastic materials (ISO7619-1:2010). Dimensions of the test specimens and testing procedure were done according to (ASTM D 2240-05). Ten samples were tested for each of the three materials using Shore hardness apparatus. For elastic modulus test, thermoplastic sheets that are less than 1mm in thickness are measured according to the ASTM D 882-02 and those that are 1mm or more are measured according to the ASTM D 638-02a. Ten samples were tested for each of the three materials and the elastic modulus was calculated using tensile test in the Universal testing machine. Results: Hardness test for the three brands showed no significant differences among them. Leone 0.8mm was less than Duran 1mm and Clear aligner 0.5mm in the elastic modulus. In conclusion: There is no significant difference in hardness among all the three materials used in this study. The elastic modulus of Duran 1mm and Clear aligner 0.5mm showed no significant difference and both are significantly higher than Leone 0.8mm